The Cambrian period of the Paleozoic era was characterized by a diverse eruption of life in the seas. Life had not yet evolved on land. The highest forms of life existing at this time were the trilobites, which reached lengths up to two feet. Green and red algae started to appear as well as sponges, gastropods, and segmented worms.

The Cambrian is the earliest period in whose rocks are found numerous large, distinctly fossilizable multicellular organisms that are more complex than sponges or medusoids. During this time, roughly fifty separate major groups of organisms or "phyla" (a phylum defines the basic body plan of some group of modern or extinct animals) emerged suddenly, in most cases without evident precursors. This radiation of animal phyla is referred to as the Cambrian explosion.

The Cambrian Explosion is the commonly used term to denote the radiation of animal phyla that started about 570 million years ago which is 30 million years before beginning of the Cambrian geologic period and proceeded through the Cambrian. The Varangian glaciation, which preceded this radiation, along with subsequent greenhouse warming of the Earth, is theorized to have provided the evolutionary pressure. It is thought that severities in climate led to the "invention" of sexual reproduction which increased the rate of evolutionary change.

Evidence for the earlier animal forms that may have been the precursors of this radiation date from 600 million years. Notable among these are trace fossils in the form of imprints of odd animals and their activity which paleontologists call the Ediacaran fauna. These organisms were soft-bodied and are found with various strange body forms. Small shelly organisms of this period also included cloudinia - an extinct animal phylum that formed small Late Proterozoic tubelike or conical fossils consisting of "cup-in cup" segments of calceraecous material.